In some applications such as a difference in time of arrival (DTOA) circuits, measurement of the difference in time of arrival of two signals at respective input terminals is required. Such circuits require that the first arriving signal be applied to one given terminal and that the second arriving signal be applied to the remaining terminal. For description of one such DTOA circuit, reference is made to "The Vernier Time-Measuring Technique" by Robert G. Baron, Proceedings of the IRE, January 1957, pp. 21-30.
In some situations either of the two input signals may be the first to arrive. Therefore, a circuit must be placed ahead of the DTOA circuit to detect the arrival of the first arriving signal and direct an indication of its arrival to the one given terminal of the DTOA circuit and to detect the arrival of the second arriving signal and direct an indication of its arrival to the remaining DTOA terminal.